This invention relates to data transmission systems. In its first aspect it is concerned with systems in which a number of transmission links are connected in sequence. Such systems have been used, for example, in networks for connecting computer processors and high-speed peripherals, especially with the communicating devices being arranged in a ring or loop. Systems of this latter kind are described for example in "An introduction to local area networks" by D. D. Clark, K. T. Pogran and D. P. Reed in Proceedings of the IEEE, November 1978, pages 1497 to 1516.
The transmission links are commonly connected to apparatus which passes data to or from an associated device which wishes to communicate over the system. In this apparatus it is usually necessary to extract a clock signal which is also used in retransmitting data onto the next transmission link. In this manner a chain of clock signals are derived, and it is found that at the end of a long sequence timing difficulties can arise which limit the number of consecutive stages possible. For example, if phase-locked loops are used each stage produces a residual clock jitter which can accumulate from stage to stage. The difficulties are especially great if the system is connected back on itself in a ring.